Printers and scanners having an automatic document feeder are two examples of imaging systems in which a media advance system moves successive sheets of media from a media input holder toward a region of the imaging system where imaging is performed, such as printing of an image or scanning of a document. In order to achieve fast throughput, it is desired to avoid long delays between the advancing of successive sheets.
Some imaging systems include a sheet registration mechanism or other mechanism that interrupts the continuous end-to-end feeding of successive sheets of media. For example, commonly assigned US Patent Application Publication No. 2009/0174733, incorporated herein by reference in its entirety, describes an inkjet carriage printer having two selectable modes of media advance—a tailgating mode in which a subsequent sheet immediately follows a previous sheet with no gap in between, and a deskew mode for aligning sheets of media which may otherwise be skewed during media advance. In the tailgating mode all media advance rollers, including a pick roller, a turn roller, a feed roller and an output roller are all continuously rotated in a forward direction. When a sheet of media in the media input holder has been advanced past the pick roller, the pick roller comes into contact with the next sheet of media and immediately begins to advance it. Such a print mode facilitates fast throughput of successive sheets, but is incapable of dynamically aligning sheets which are skewed during media advance. In the deskew mode, the pick roller advances the media to the turn roller, and the turn roller advances the media to the feed roller which is rotated in reverse direction. If a corner of the leading edge of a skewed sheet of media reaches the reversely rotating feed roller prior to the rest of the leading edge, it is temporarily prevented from further advancement, allowing the rest of the leading edge to catch up, thus deskewing the sheet. The feed roller and the output roller (which is geared to always move in the same direction as the feed roller) are then rotated in the forward direction to move the sheet through the print region and into the media output holder. In the deskew mode, power is not applied to the pick roller until after the printing of the entire page has completed. This helps to ensure that media collisions and paper jams will not occur at the feed roller as a previous sheet is moved backward by the output roller and a subsequent sheet is moved forward by the turn roller. However, this results in a significant time delay between the feeding of successive sheets, thereby slowing down printing throughput.
U.S. Pat. No. 5,415,387 discloses a sheet feed device for a selectable print speed image forming device having a time-delayed pick roller and a sheet registration mechanism. However, in that apparatus, rotation of the pick roller is independently controlled by an electromagnetic clutch, adding to the complexity and cost of the media advance system. U.S. Pat. No. 6,446,954 discloses an automatic document feeder for a scanner including a delay gear between the main gear and the pick roller. The main gear also provides power to a drive roller (which is analogous to a feed roller). A slip clutch on the pick roller shaft prevents the pick roller from rotating in reverse when the drive roller is driven in reverse. Such an apparatus would require that the drive roller (or feed roller) complete its reverse motion before the pick roller could advance the next sheet.
What is needed is a simple, low-cost media advance system and method of operation that can controllably provide a delay between picking of successive sheets of media, such that the delay is long enough to prevent media collisions at the feed roller during a media deskewing operation, and short enough to be consistent with fast throughput of successive sheets.